1. Field of the Invention
The present invention relates to a radiation image intensifier for converting a radiation image into a visible light image or an electrical image signal and to a method of manufacturing the same. Note that a radiation beam, serving as a target of the present invention, for exciting an input screen is a radiation beam, in a wide sense, including X-rays, .alpha. (alpha)-rays, .beta. (beta)-rays, .gamma. (gamma)-rays, a neutron beam, an electron beam, heavily charged particle beam, or the like.
2. Description of the Related Art
As a typical radiation image intensifier, an X-ray image intensifier will be described below. The X-ray image intensifier is useful to examine the internal structure of a human body or an object and is used to convert, into a visible light image or an electronic image signal, a radiation image from a fluoroscopy system or a radiograph system for examining the transmission concentration distribution of a radiation beam radiated on the human body or the object.
An X-ray image intensifier is demanded to efficiently convert an X-ray image into a visible light image or an electrical image signal while the contrast or resolution of the X-ray image is kept with a sufficient fidelity. In practice, the degree of fidelity depends on the performances of the constituent elements in the X-ray image intensifier. In particular, since an X-ray input part has conversion characteristics inferior to those of the output screen part, the degree of fidelity of an output image largely depends on the characteristics of the input screen part. In the structure of the input screen part which is conventionally used, i.e., in a structure in which a thin aluminum substrate is arranged inside the X-ray incident window of a vacuum vessel and a phosphor layer and a photocathode layer serving as an input screen adheres to the rear surface of the substrate, a total transmittance of X-rays incident on the input screen becomes low, and X-rays are frequently scattered at the incident windows. For this reason, characteristics having a sufficiently high contrast and a sufficiently high resolution cannot be easily obtained.
Therefore, a known structure for causing an input screen constituted by a phosphor layer and a photocathode layer to directly adhere to the rear surface of the X-ray incident window of the vacuum vessel had been already described in Jpn. Pat. Appln. KOKAI Publication No. 56-45556 or European Pat. Appln. KOKAI Publication No. 540391A1. In such a structure, the X-ray incident window of a vacuum vessel has a substrate which permits the X-rays to penetrate therethrough. For this reason, a decrease in transmittance with respect to incident X-rays and scattering of X-rays can be suppressed, and characteristics having a relatively high contrast and a relatively high resolution can be obtained.
The shape of the input screen constituted by a phosphor layer and a photocathode layer is designed as to be curved shape optimal to minimize deformation of an image plane formed on an output screen by an electron lens system. For this reason, the shape of the input screen is designed to be a paraboloid or a hyperboloid more frequently than a shape having a single radius of curvature.
Although a structure for causing an input screen constituted by a phosphor layer and a photocathode layer to directly adhere to the rear surface of the X-ray incident window of a vacuum vessel is widely known as a technique, this structure is not in practical use. A main reason why the structure is not used in practice is as follows. That is, since the X-ray incident window is deformed by the atmospheric pressure, the input screen does not stably adhere to an X-ray incident window of the vacuum vessel, or an image plane formed by an electron lens system is easily deformed. In a general X-ray image intensifier, even when an electron lens system including an input screen is optimally designed, when the input screen is partially deformed and moved on the vacuum side or the atmospheric pressure side by, e.g., 0.5 mm, a satisfactory output image cannot be obtained due to the deformation of the electron lens system.
Note that, in order to obtain a high resolution and high X-ray detection efficiency, the input screen, especially, the X-ray exciting phosphor layer is formed by vacuum deposition to have a small columnar crystal structure having a relatively large thickness. However, in a method of performing vacuum deposition such that an x-ray incident window is inserted into a film forming apparatus, the crystal structure of an obtained phosphor layer is largely influenced by the substrate temperature of the X-ray incident window. For example, a phosphor layer consisting of sodium-activated caesium iodide (CsI) is deposited to have a thickness of about 400 .mu.m. For this reason, an increase in substrate temperature caused by heat of sublimation or heat radiated from an evaporation unit when the evaporated material adheres to the substrate of the incident window cannot be neglected. When the phosphor layer having a desired thickness is to be formed within a short time, the substrate temperature abruptly increases, satisfactorily thin columnar crystal grains cannot be obtained. When the thickness of the incident window is make thinner to increase the transmittance with respect to incident X-rays, an increase in substrate temperature of the incident window during formation of the phosphor layer becomes conspicuous. For this reason, satisfactorily thin columnar crystal grains cannot be obtained.
In order to avoid the above problems, an amount of material adhering to the substrate may be decreased per unit time. However, in this case, a deposition time required for depositing the layer having a desired thickness becomes very long. Therefore, this method is not practical.